Solutions based on brakeless stepper motors have conventionally been used as an essential part of the operating machinery of electronic pipettes. The combination presented in Finnish patent no. 87740 including an electronic brake, a DC-motor and a closed loop actuator structure with an encoder has proven to be a good one. With such a construction, a better overall outcome for the operation, size and weight of the pipette is accomplished, compared to solutions involving a stepper motor. Good properties achieved with this structure are, among others, the ability to stop the motion rapidly, and an advantageous size-weight/speed-power ratio. The brake device mentioned in FI 87740 is implemented with a spring return solenoid structure. The problem with this brake structure is that the power consumption is high, which is a disadvantageous feature in a battery powered pipette. To keep the brake open, a current must be fed to the solenoid and for speeding up the brake and for speeding up its movement, the power of its spring has to be increased, which increases the operational power consumption.
The brake mechanism according to the present invention solves the above mentioned problems. The brake actuator according to the invention is used for abrupt stopping of the movement of the actuator that moves the plunger. The brake actuator according to the invention comprises an electromotive functional member that turns a brake part stopping the rotational movement by means of a mediating mechanism.
The invention is directed to a brake mechanism for use in an electronic pipette, which mechanism is used for abrupt stopping of the movement of the actuator that moves the plunger. The mechanism comprises a rotatable, essentially circular rotor which is provided at its periphery with brake notches arranged at regular intervals, and a brake actuator comprising an actuating member, a brake member to which a brake cam compatible with the brake notches is connected, a mediation mechanism for transmission of the movement of the motion member to the brake member, the brake member comprising a turnable member connected to the motion member that is arranged to act with the brake member such that the movement of the turnable member can be accelerated before it essentially effects the movement of the brake member.
According to a preferred embodiment the brake member comprises an opening in which the turnable member is adapted, here a lever (2) that is turnably adapted on a shaft (2a). The shape of the opening is arranged to be such that the turnable member can turn between two locked end positions. Preferably, the turnable member turns about 180° from one end position to the other. The size of the opening in relation to the turning lever is such that it enables the turning lever to move between the two positions and that depending of the position of the turning lever in the opening, the turning lever forces the brake member to turn around its axis from a locked position to a free position, and from a free position to a locked position.
According to a preferred embodiment the brake notches of the rotor comprise protrusions for stopping the rotor and slots for locking the rotor in relation to the brake cam. The protrusions in the rotor stop the rotor as the brake cam locks into a slot between two protrusions. Depending on the direction of rotation, the brake cam ends up in the upper or lower brake notch of FIG. 3a. When the rotor rotates counter-clockwise, the brake cam locks into the lower notch and when it rotates clockwise in turn into the upper notch.
In the brake configuration according to the invention, the drive means can be a DC-motor, a linear solenoid or a rotary solenoid.
In the configuration according to the invention the brake drive means is coupled to the brake member by a mediation mechanism. The configuration according to the invention comprising a mediation mechanism has the first advantage of the brake part locking into both end positions. A second advantage of the solution according to the invention is the high momentary velocity of the brake part physically stopping the movement, i.e. the turnable member. The mediation member between the brake drive means, i.e. the movement-inducing means (the motor), and the brake member enables the brake motor to accelerate before the movement for stopping the brake member rotor begins. Thus, the turning movement of the turnable member reaches its top speed almost immediately as the movement begins, whereby the brake member hits its target location in the rotating rotor in a more controlled manner. On the other hand, the high velocity of the brake member enables, in terms of functionality, a preferred higher velocity of the stopping part at the braking moment. A third advantage is that due to the kinetic energy accumulated during the acceleration of the drive means, force is gained in the movement of the brake member in the very beginning of the movement, which leads to more reliable opening and closing of the brake.
The mediation mechanism functionality according to the invention can be implemented with a crank mechanism or a cam gear mechanism. In both mechanisms the brake actuator turns approximately 180 degrees in all, and the brake member stopping the movement turns a fraction of this. In a cam gear mechanism, the brake member and the brake actuator are coupled to each other with a connecting rod. In this mechanism the brake part reaches its top angular velocity between the two ends of the movement of the brake actuator, the velocity is zero or close to zero at the end positions. According to a preferred embodiment, the actual brake member stopping the rotation movement is moved by a cam gear mechanism that is simpler than a crank mechanism.
An advantage of the invention over the prior art is also that in the structure according to the invention, the brake locks the rotor into place without any clearance causing a movement error.
A significant advantage of the structure according to the invention is the ability of the brake to lock into the extreme positions, preventing undesired closing of the brake during running in case of a loss of electricity. This is a very important feature since braking while running at high speed may break the operating mechanism.
According to the invention the brake motor turns the brake member back and forth by means of the mediation mechanism. In a preferred embodiment the brake member turns about 20°. At one end the “barb” of the brake member locks the rotating movement of the rotor and at the other end the rotation is free.
In FIGS. 1-4, one technical solution for implementing the function according to the invention is presented. FIG. 1 shows the brake member (1) that is attached to the body of the brake at its pivot pin (1b) such that it can turn. The task of the brake cam (1a) of the brake member is to stop the rotation of rotor (3) by setting into a brake notch (3a) of the rotor. The turning lever (2) is locked onto the shaft (2a) of the electric motor (not shown) and is so located that at the extreme ends of its movement the brake member is either open or closed. In the situation in FIG. 1, the brake is closed and the relative location and the geometry of the brake member and the turning lever locks the brake at the position even though the motor is not powered on.
In FIG. 2, the motor is switched on and the turning lever has accelerated freely approximately 90° and is about to hit the brake member.
In the situation of FIG. 3, the turning lever has turned to its other end position and has at the same time turned the brake member into its open position. Also in this position, the brake is locked into position as in FIG. 1.
In FIG. 4 we are in the situation of FIG. 2 of applying the brake.
In FIG. 5, the power generation and speed advantage achieved by the mechanism are illustrated. The X-axis represents time and the Y-axis the angular velocity. When the motor is turned on at time t1, it accelerates according to graph A. Graph B illustrates the angular velocity of the brake part. At time t2 the turning lever hits the brake member at high speed, quickly turning it to its other end position. At time t3 the movement of the parts stops and the brake is locked at the other end position.